Alex Clemmer is a computer programmer. Other programmers love Alex, excitedly describing him as "employed here" and "the boss's son".
Alex is also a Hacker School alum. Surely they do not at all regret admitting him!
(My batch ended on August 22, 2013, but as they say, never graduate.)
When I was reverse engineering the Snapchat API, I spent a fair amount of time wondering if there was a quick way to prototype HTTP requests. After complaining to a few friends, it turns out that there is: the
telnet utility on unix systems.
Goal: Let’s see how we can use
telnet to issue an HTTP request that is equivalent to typing
www.google.com into your address bar and pressing enter.
Aside: Before we begin, it’s worth noting that different parts of the URL are used by the server and the client. If we were accessing
www.google.comis used to figure out which server to send the request to, while
/somepage.htmlis telling the Google server which page to serve up. How this works specifically is outside the scope of this article, but knowing that it works might help you understand this article.
Let’s start by opening a request to the Google servers. The form of request for opening a connection using
$ telnet <hostname> <port>
The word host is just network-speak a “machine that exists on a network”. A hostname is something that identifies a machine on a network — perhaps a URL or an IP address (see my previous exploration of IP addresses here). In this case, the network is the global IP Internet, and the hostname could be something like
The last part of the command involves supplying a port. A port is just a 16-bit number that is used to help uniquely identify connections to some server — I won’t get into why this helps now, but it turns out that connecting to a server to do HTTP requests usually involves connecting to port 80, which is what we will do here.
So let’s connect to the Google server using
$ telnet www.google.com 80 Trying 126.96.36.199... Connected to www.google.com. Escape character is '^]'.
From here we are free to type an HTTP request in plaintext, which will get sent to the server. In the following example, I’ll grey out the old stuff:
$ telnet www.google.com 80 Trying 188.8.131.52... Connected to www.google.com. Escape character is '^]'. GET / HTTP/1.1 Host: www.google.com
An HTTP request consists of three things:
GET / HTTP/1.1
<header name>: <header data>; here
Host: www.google.comis an example of a request header. There are other types of header other than
Host, but we don’t really care about them right now.
The request line
GET / HTTP/1.1 has a general form
<request type> <uri> <HTTP version>.
The request type in this case is a GET, which just tells the server to generate and return the resource at the
<uri>. There are also other request types, like POST, but right now we just care about fetching webpages from URLs.
The URI is the resource we’d like to retrieve from the server. In our case it’s
/, the root URL, or the thing that you see any time you visit Google. It could have been
/search?q=cows, which is the string we would enter to request that Google search the Internet for the word “cows”.
The HTTP version is the version of HTTP we’re using. You should use version 1.1, but old applications use 1.0.
After you type in that last empty line, the server will respond:
An HTTP response looks remarkably like an HTTP request! They consist of:
HTTP/1.1 200 OK
<header name>: <header data>(i.e., exactly like the request header)
A response line is always of the form
<HTTP version> <status code> <status message>. In this case we see that the server has responded in HTTP version 1.1, with status code 200, and status message “OK”. Code 200 indicates that the request has executed correctly, and the message “OK” tells us the same in English. There are many other codes with many different meanings, but those are for another post.
And with that, you’ve successfully executed a GET request to Google.
The cursor is still blinking at the end, though, because you are free to continue typing requests in.
 Note that there is also a Telnet protocol. But in this article we’ll mainly talk about
telnet the unix utility.